This invention relates to the fabrication of devices on thin, rigid materials. More particularly, this invention relates to the manufacture of integrated circuits on wafers of silicon or other semiconductor material.
The explosion in the electronics industry over the last several decades has largely been fueled by manufacturing techniques allowing for the construction of millions of microscopic electronic circuit elements onto integrated circuits made up of layers of silicon, other metals, semiconductor materials, and insulation materials. Typically, microelectronic circuits are fabricated on thin wafers of silicon measuring approximately five to eight inches in diameter and having a thickness of approximately 20 mils. Integrated circuits may be manufactured by a variety of processes including photolithography, epitaxial deposition, ion implantation, etc. These processes create layers of different materials on top of the silicon wafer substrate and in these layers are formed various circuit elements. Typically, a number of identical integrated circuits are fabricated at the same time on a single silicon wafer and then the wafer is cut or scored into a number of dies, each die containing one device. FIG. 1 depicts a silicon wafer 10 of the prior art showing scoring lines 11 dividing the wafer into a number of dies 12. A die such as 12a typically contains one integrated circuit. After fabrication and scoring, the die is typically packaged in a housing made of a plastic or ceramic molding compound. The die package contains leads allowing the packaged die (also referred to as a "chip") to be interconnected with other chips in an electronic apparatus such as a computer. The die is connected to the leads by a number of wire bonds, each wire bond connecting a pad on the die to a lead.
While a die such as 12a is primarily an electrical apparatus, its physical characteristics may be very important in its overall reliability. One problem that has commonly arisen in the manufacture of integrated circuit dies is the problem of cracking of the metal, semiconductor, and passivation layers. The cracking phenomenon is often observed during temperature cycling used to test manufactured packaged integrated circuits. The cracking affects electrical reliability of the integrated circuits by causing shorts or current leakage failures. Failure analysis has shown that the metal layers are cracked especially in the die corners. It is believed that cracking occurs as a result of the direct interaction of the die surface with the molding compound that makes up the packaging, the driving force being the stresses arising from the large mismatch in coefficients of thermal expansion among the adjacent materials.
One method that has been used to reduce or prevent die cracking is use of a gel die coating. This gel may be a compound such as silicone-vinyl or silicone-hydride. The gel is placed on the die and adhesed prior to the die being encased in the chip packaging. The gel absorbs stresses that arise due to the different thermal expansion of the die material and the material that makes up the chip packaging.
While use of die coat has been found to greatly reduce cracking, the die coat creates other problems. Chief among those is that interaction between the coat, the pads and the wire bonds may break some of the wire bonds, again reducing chip reliability. Use of die coat also adds additional process steps to the chip manufacture, which increases the total chip cost.
What is needed is an improved die structure and a method for preventing cracks that form in the metal and passivation layers on an integrated circuit die.